Successful marrow transplantation requires not only genetically compatible donors but the elimination of normal or neoplastic hematopoietic cells of the recipient without destruction of other vital tissues. Among the factors influencing the effectiveness of radiation treatment of the hematopoietic system was exposure rate and direction of the exposure. Simultaneous two-directional exposures were most effective. Exposure rate reduced the effec- tiveness of dorsal exposures while exposure rate plus lack of homogeneity further reduced the effectiveness of ventral exposures. Continuous reciprocal alternate exposures were used to establish the time required to initiate repair when a reduced exposure rate was used. Compliance with the requirement of the Animal Welfare Act that sedation be used to eliminate stress during treatment may reduce the biological effectiveness of the radiation treatment and the success of the bone marrow transplantation. Other physical factors may also affect the success of bone marrow transplantation but have proved to be less readily investigated.